Solar Cell I-V Curve | mysimulator.uk

A solar cell is a diode with a light input

At its heart a photovoltaic cell is a p-n junction — the same basic device as a rectifying diode — with one extra ingredient: photons striking the semiconductor with enough energy knock electrons across the bandgap, generating electron-hole pairs that the junction's built-in electric field separates before they can recombine. That separated charge is current you can draw out of the device. The complete behaviour, dark and lit, is captured by the Shockley single-diode model:

I = I_L - I_0 · [ exp(qV / (nkT)) - 1 ] - V / R_sh

I_L  = photogenerated current (∝ irradiance)
I_0  = diode reverse-saturation (dark) current
q    = electron charge, k = Boltzmann constant, T = temperature
n    = diode ideality factor (≈1-2)
R_sh = shunt resistance (leakage path), often folded in with series resistance R_s

live demo · I-V and P-V curves reshaping under irradiance and temperature● LIVE

Reading the I–V curve

Plot current against voltage and two extremes stand out: short-circuit current (Isc), the current at V = 0, which is essentially I_L itself since the diode and shunt terms vanish; and open-circuit voltage (Voc), the voltage at I = 0, where the photogenerated current is entirely cancelled by the diode's forward-bias leakage. Between these two points the curve traces a soft knee, and it is the shape of that knee — not just the two endpoints — that determines how much usable power the cell delivers.

Power, fill factor and the maximum power point

Power is simply P = I·V, so the P–V curve is zero at both Isc and Voc and peaks somewhere in between — that peak is the maximum power point (MPP), and every real solar installation uses an MPPT (maximum power point tracking) controller to keep operating exactly there as irradiance and temperature drift throughout the day. How close the actual power peak gets to the theoretical ceiling of Isc × Voc is the fill factor:

FF = (I_mpp · V_mpp) / (Isc · Voc)
PCE (power conversion efficiency) = (I_mpp · V_mpp) / P_in = FF · Isc · Voc / P_in

A fill factor near 1 means a sharply squared-off I-V knee — an ideal cell with negligible series resistance and no shunt leakage. Real silicon cells achieve fill factors around 0.75–0.85; series resistance rounds off the knee near Voc, and shunt leakage droops the flat plateau near Isc, both of which pull FF down and cost real power.

Why irradiance and temperature push the curve in opposite directions

Isc scales almost linearly with irradiance, since photogenerated current is directly proportional to the rate of photon absorption — halve the sunlight and Isc roughly halves. Voc responds far more weakly, only logarithmically (it enters through the exp term in the diode equation), so on a cloudy day current collapses much faster than voltage does. Temperature has the opposite signature: rising temperature increases the intrinsic carrier concentration and hence I_0, which pushes Voc down noticeably (a typical silicon cell loses roughly 0.3-0.4% of Voc per °C), while Isc actually rises slightly with temperature due to bandgap narrowing. The net effect is that hot solar panels produce measurably less power than cool ones at the same irradiance — a real efficiency penalty that panel datasheets quote as a temperature coefficient.

The band diagram underneath it all

The p-n junction's built-in field comes from band bending at the junction: majority carriers diffuse across until the resulting space charge creates an electric field strong enough to stop further net diffusion, leaving a depletion region with a built-in potential. A photon absorbed within or near this depletion region generates an electron-hole pair that the field immediately sweeps apart — electron toward the n-side, hole toward the p-side — before they can recombine. Photons absorbed too far from the junction generate pairs that mostly recombine before they can be collected, which is why cell thickness and minority-carrier diffusion length are core design parameters, not incidental details.

Frequently asked questions

What is the difference between fill factor and efficiency?

Fill factor measures how square the I-V knee is — the ratio of actual maximum power to the theoretical ceiling of Isc times Voc. Power conversion efficiency measures how much of the incoming sunlight's power actually comes out as electricity; it depends on fill factor but also on Isc, Voc and the incident irradiance itself.

Why does a solar panel produce less power on a hot day even with full sun?

Open-circuit voltage falls measurably as temperature rises — roughly 0.3-0.4% per °C for silicon — because the diode's dark saturation current grows with temperature. Short-circuit current rises only slightly in comparison, so the net effect is a real drop in maximum power output even though irradiance hasn't changed.

Why do solar inverters need maximum power point tracking?

The I-V curve's power peak (the MPP) sits at a voltage that shifts continuously with irradiance and temperature, neither of which stay constant over a day. An MPPT controller continuously adjusts the operating voltage to track that moving peak, rather than running the panel at a fixed voltage where it would rarely deliver its full available power.

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The Solar Cell I–V Curve, Explained